①. 坏境准备
CREATE TABLE `employees` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(24) NOT NULL DEFAULT '' COMMENT '姓名', `age` int(11) NOT NULL DEFAULT '0' COMMENT '年龄', `position` varchar(20) NOT NULL DEFAULT '' COMMENT '职位', `hire_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入职时间', PRIMARY KEY (`id`), KEY `idx_name_age_position` (`name`,`age`,`position`) USING BTREE ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COMMENT='员工记录表'; INSERT INTO employees(name,age,position,hire_time) VALUES('LiLei',22,'manager',NOW()); INSERT INTO employees(name,age,position,hire_time) VALUES('HanMeimei', 23,'dev',NOW()); INSERT INTO employees(name,age,position,hire_time) VALUES('Lucy',23,'dev',NOW()); -- 插入一些示例数据 drop procedure if exists insert_emp; delimiter ;; create procedure insert_emp() begin declare i int; set i=1; while(i<=100000)do insert into employees(name,age,position) values(CONCAT('zhuge',i),i,'dev'); set i=i+1; end while; end;; delimiter ; call insert_emp();
②. 覆盖索引、索引下推
- ①. 联合索引第一个字段用范围不会走索引
(结论:联合索引第一个字段就用范围查找不会走索引,mysql内部可能觉得第一个字段就用范围,结果集应该很大,回表效率不高,还不如就全表扫描)
EXPLAIN SELECT * FROM employees WHERE name > 'LiLei' AND age = 22 AND position ='manager'
②. 强制走索引
结论:虽然使用了强制走索引让联合索引第一个字段范围查找也走索引,扫描的行rows看上去也少了点,但是最终查找效率不一定比全表扫描高,因为回表效率不高
EXPLAIN SELECT * FROM employees force index(idx_name_age_position) WHERE name > 'LiLei' AND age = 22 AND position ='manager';
-- 做了一个小实验: -- 关闭查询缓存 set global query_cache_size=0; set global query_cache_type=0; -- 执行时间0.333s SELECT * FROM employees WHERE name > 'LiLei'; -- 执行时间0.444s SELECT * FROM employees force index(idx_name_age_position) WHERE name > 'LiLei';
③. 覆盖索引优化
EXPLAIN SELECT name,age,position FROM employees WHERE name > 'LiLei' AND age = 22 AND position ='manager';
④. in和or在表数据量比较大的情况会走索引,在表记录不多的情况下会选择全表扫描
EXPLAIN SELECT * FROM employees WHERE name in ('LiLei','HanMeimei','Lucy') AND age = 22 AND position ='manager'; EXPLAIN SELECT * FROM employees WHERE (name = 'LiLei' or name = 'HanMeimei') AND age = 22 AND position ='manager';
-- 做一个小实验,将employees 表复制一张employees_copy的表,里面保留两三条记录 EXPLAIN SELECT * FROM employees_copy WHERE name in ('LiLei','HanMeimei','Lucy') AND age = 22 AND position ='manager'; EXPLAIN SELECT * FROM employees_copy WHERE (name = 'LiLei' or name = 'HanMeimei') AND age = 22 AND position ='manager';
⑤. like KK% 一般情况都会走索引
EXPLAIN SELECT * FROM employees WHERE name like 'LiLei%' AND age = 22 AND position ='manager'; EXPLAIN SELECT * FROM employees_copy WHERE name like 'LiLei%' AND age = 22 AND position ='manager';
⑥. 这里给大家补充一个概念,索引下推(Index Condition Pushdown,ICP), like KK%其实就是用到了索引下推优化,什么是索引下推了？
补充说明:
(1). 对于辅助的联合索引(name,age,position),正常情况按照最左前缀原则,SELECT * FROM employees WHERE name like ‘LiLei%’ AND age = 22 AND position
=‘manager’ 这种情况只会走name字段索引,因为根据name字段过滤完,得到的索引行里的age和position是无序的,无法很好的利用索引。
(2). 在MySQL5.6之前的版本,这个查询只能在联合索引里匹配到名字是 ‘LiLei’ 开头的索引,然后拿这些索引对应的主键逐个回表,到主键索引上找出相应的记录,再比对age和position这两个字段的值是否符合
(3). MySQL 5.6引入了索引下推优化,可以在索引遍历过程中,对索引中包含的所有字段先做判断,过滤掉不符合条件的记录之后再回表,可以有效的减少回表次数。使用了索引下推优化后,上面那个查询在联合索引里匹配到名字是 ‘LiLei’ 开头的索引之后,同时还会在索引里过滤age和position这两个字段,拿着过滤完剩下的索引对应的主键id再回表查整行数据
(4). 索引下推会减少回表次数,对于innodb引擎的表索引下推只能用于二级索引,innodb的主键索引(聚簇索引)树叶子节点上保存的是全行数据,所以这个时候索引下推并不会起到减少查询全行数据的效果。
⑦. 为什么范围查找Mysql没有用索引下推优化？
估计应该是Mysql认为范围查找过滤的结果集过大,like KK% 在绝大多数情况来看,过滤后的结果集比较小,所以这里Mysql选择给 like KK% 用了索引下推优化,当然这也不是绝对的,有时like KK% 也不一定就会走索引下推
